To process each element of a list of things. “Hold on, I've
got to loop through my paper mail.” Derives from the
computer-language notion of an iterative loop; compare cdr down (under cdr),
which is less common among C and Unix programmers. ITS hackers used to say
IRP over after an obscure pseudo-op
in the MIDAS PDP-10 assembler (the same IRP op can nowadays be found in
Microsoft's assembler).